The invention relates to an injection nozzle for fuels, such as finds application, for example, for injecting fuel into combustion chambers of internal-combustion engines.
Injection nozzles for fuels, in particular for injecting fuel under high pressure into combustion chambers of internal-combustion engines, have long been known from the state of the art. Accordingly, a fuel injector with an injection nozzle is known from DE 199 36 668 A1, wherein the injection nozzle has a nozzle body with a pressure chamber formed therein. Arranged in longitudinally displaceable manner in the pressure chamber is a piston-shaped nozzle needle which has a sealing surface at one end, with which it interacts with a nozzle seat formed in the nozzle body for the purpose of opening and closing at least one injection port. For the purpose of controlling the longitudinal motion of the nozzle needle, at the end situated opposite the nozzle seat a control chamber has been formed which can be filled with fuel under high pressure and in which, via a control valve, a variable fuel pressure can be set by which a closing force can be exerted on the nozzle needle in the direction of the nozzle seat. The pressure chamber is connected to a fuel reservoir in which fuel is held under high pressure, in order to supply the pressure chamber with fuel under constant high pressure at all times.
The sealing of the injection ports by the resting of the nozzle needle on the nozzle seat represents the closed state of the injection nozzle. If fuel is to be injected into a combustion chamber, the nozzle needle is moved away from the nozzle seat in the longitudinal direction, by the hydraulic pressure in the control chamber being lowered. The hydraulic forces in the pressure chamber thereupon move the nozzle needle away from the nozzle seat, and the injection ports are released from the nozzle needle, so that fuel is ejected from the pressure chamber through the injection ports. In this process it is important for a clean injection that the nozzle needle moves away from the nozzle seat very rapidly. If it does so only slowly, a throttle gap forms between the sealing surface of the nozzle needle and the nozzle seat, through which fuel flows out of the pressure chamber to the injection ports only with reduced pressure, so that this fuel is only inadequately atomized when it emerges from the injection ports. Accordingly, this so-called seat-throttle region has to be kept as short as possible by a rapid movement of the nozzle needle, in order to increase the effective injection pressure at the injection ports rapidly to the level within the pressure chamber in order to obtain a good atomization of the fuel. Insufficiently atomized fuel otherwise results in insufficient combustion within the combustion chamber, and hence in increased hydrocarbon emissions of the internal-combustion engine.
For the purpose of increasing the needle-opening speed, the pressure in the control chamber can be lowered as rapidly as possible. This can be obtained by the outflow throttle, via which the fuel can flow away out of the control chamber, being configured with a large cross-section of flow in relation to the inflow throttle via which the control chamber is filled with fuel under high pressure. If the control chamber is additionally also filled via the outflow throttle, by the outflow throttle being connected to the high pressure with the control valve closed, any enlargement of the throttles results in a faster build-up of pressure or reduction of pressure. However, a rapid drop in pressure or build-up of pressure impairs the capability of the injection valve to handle extremely small amounts, since as a result the injected quantity of fuel reacts very sensitively to the actuation-time of the control valve. This entails a large stroke-to-stroke scatter—that is to say, a greater stochastic scattering of the injected quantity around the desired value from injection to injection.
Furthermore, a certain limit is set to the speed of the drop in pressure within the control chamber by virtue of the fact that in many applications the nozzle needle is operated in the so-called ballistic mode in which the nozzle needle does not reach a mechanical stroke stop but is retarded prior to reaching a stroke stop by renewed rise in pressure within the control chamber and is accelerated back in the direction of the nozzle seat. However, if the pressure in the control chamber drops too rapidly, this ballistic mode can no longer be realized, since the nozzle needle reaches the mechanical stroke stop prematurely by reason of its high opening speed.